1. Field of the Invention
The present invention relates to a sound localization control apparatus and method for controlling localization of sound image, i.e., localization of sound sources as perceived by the human ear.
2. Prior Art
Recently, there has been renewed interest in a technique known as the "binaural sound" technique which recreates for the listener a real and dynamic stereo sound image. The reason for this renewed interest in the binaural technique is due to its enormous potential for application in large screen television and "virtual reality". The binaural technique has been made possible through the recent development of digital signal processing.
In the sound reproduction of binaural signals, headphones are generally used. The use of headphones is generally accepted due to the wide use of the headphone stereo. However, as will be described below, there are cases in which listeners prefer to listen to the reproduction of sound through the use of loudspeakers.
In the case where binaural signals are reproduced through two loudspeakers positioned at the left side and right sides of a listener, the sound emanating from one of the loudspeakers propagates to the left and right ears whereby a "cross-talk" phenomena is established. There is a problem in that the listener cannot perceive the localization of sound image of the original sound expressed in the binaural signals due to the effect of cross-talk. In order to overcome this problem, a method is proposed in which a pre-process is carried out on the binaural signals to be reproduced; and the results of the pre-process are reproduced through the left and right sides loudspeakers to cancel the effect of cross-talk. Hereinafter, a detailed description of the method will be given.
In a general listening room, there are many reflection sounds and the model for reviewing the method is quite complex. For this reason, the description will be given with respect to the model of sound reproduction in a non-reverberation room. In the case where sounds are emanated from two loudspeakers positioned at the left and right sides of a listener in the non-reverberation room, the sound transmission, in Which the sound emanates from the left and right loudspeakers and propagates to the left and right ears of the listener, is simulated by the model shown in FIG. 6.
In FIG. 6, Hrr designates a transfer function of a sound transmission path through which sound R, emanated from right loudspeaker 1, propagates to right ear 3; Hrl designates a transfer function of a sound transmission path through which sound R, emanated from right loudspeaker 1, propagates to left ear 4; Hlr designates a transfer function of a sound transmission path through which sound L, emanated from left loudspeaker 2, propagates to right ear 3; Hll designates a transfer function of a sound transmission path through which sound L, emanated from left loudspeaker 2, propagates to left ear 4. Hereinafter, the sound transmitting from right loudspeaker 1 to left ear 4, and the sound transmitting from left loudspeaker 2 to right ear 3, will be called "cross-talk components".
In this model, sound ER perceived by right ear 3 and sound EL perceived by left ear 4 are described by using the following formula (1). ##EQU2##
In the case where the listener is positioned in front of both loudspeakers such that the transfer function between listener and right loudspeaker, and the transfer function between listener and left loudspeaker, can be regarded as symmetrical, the following formulae can be used. EQU S=Hrr=Hll (2) EQU A=Hrl=Hlr (3)
The above formula (1) can be rewritten by using above formulae (2) and (3) in the following manner: ##EQU3##
In the case where matrix ##EQU4## is a regular matrix, there exists a inverse matrix of the regular matrix which is described using formula (5). ##EQU5##
If the pre-process corresponding to the inverse matrix as thus obtained is carried out on both left and right channels of audio signals to be reproduced and the processed signals are supplied to the left and right loudspeakers, the transfer function matrix corresponding to the total path through which the left and right channels of audio signals transmit to the left and right ears of the listener is described as follows: ##EQU6##
In this manner, cross-talk components can be canceled and the left and right channels of audio signals are respectively transmitted to the left and right ears without interference of one channel sound to the other.
Next, data C defined by the following formula is introduced. EQU C=A/S (7)
In this case, the following formula can be used in order to vary formula (5). ##EQU7##
The following formula (10) is obtained by applying formulae (8) and (9) to formula (5). ##EQU8##
This formula is known as Schroeder's model. The filter which performs the signal processing expressed by the inverse matrix (10) can be obtained by first measuring the transfer functions S and A of the sound transmission path, and then by calculating the value of C and 1/S based on the measurements of S and A. The desirable sound transfer function can be obtained by using the previously obtained filter since the sound transfer function between the loudspeakers and the head of the listener, which generates the cross-talk, can be corrected.
However, it is generally difficult to design a filter, which corrects the transfer function of a target system, corresponding to the inverse matrix of the transfer function matrix of the system, based on the impulse response measurement obtained from the system. Even if the design is possible, an FIR (Finite Impulse Response) filter having more than ten taps is necessary.